High resolution airborne signal processing system

ABSTRACT

A two-antenna reception system with fixed-position antennas to receive linearly frequency-modulated signals or linearly position-modulated antennas to receive fixed-frequency signals. The difference frequency generated from the antenna signals represents the angle of arrival with respect to the system baseline. Two such systems with nonparallel baselines yield angular information in space.

Lammers 5] Mar. 19, 974

[ HIGH RESOLUTION AIRBORNE SIGNAL 2,996,711 8/1961 l-leiser 343/113 R xPROCESSHNG SYSTEM 2.762.043 9/1956 Earp 343/113 DE UX Uve H. W. Lammers,5 San Mateo Dr., Chelmsford, Mass. 01824 Filed: June 27, 1972 Appl. No.:266,779

Related US. Application Data Continuation-impart of Ser. No. 833,825,June 16, 1969, abandoned.

Inventor:

US. Cl 343/113 DE, 34 3/102, 343/113 R Int. CL... .L G015 3/52 Field ofSearch 343/102, 113 R, 113 DE,

References Cited UNITED STATES PATENTS 8/1949 Hallman, Jr. 343/104Primary Examiner-Maynard R. Wilbur Assistant Examiner-Richard E. Berger[57] ABSTRACT A two-antenna reception system with fixed-positionantennas to receive linearly frequency-modulated signals or linearlyposition-modulated antennas to receive fixed-frequency signals. Thedifference frequency generated from the antenna signals represents theangle of arrival with respect to the system baseline. Two such systemswith nonparallel baselines yield angular information in space.

1 Claim, 4 Drawing Figures HIGH RESOLUTION AIRBORNE SIGNAL PROUESSTNGSYSTEM This application is a continuation-in-part of my application Ser.No. 833,825, filed June 16, 1969, now abandoned.

BACKGROUND OF THE INVENTION This invention relates generally to signalreceiving systems and more particularly to a method and apparatus toprovide angular information utilizing two antennas at fixed positions toreceive frequency swept pulses or two antennas whose relative positionis changed linearly with time while receiving fixed frequency pulses.

It has long been known that bats employ ultrasonic echo-location fortheir orientation in space. Among various techniques used in bat-sonars,one type has been found that emits constant frequency pulses which arereceived with ears in rapid relative motion. Conversely, another typeemits linearly swept signals which are received with stationary cars.

It is assumed that these techniques provide bats primarily with angularinformation as opposed to distance information that is generallyobtained by pulsing and frequency sweeping in manmade radars.

The advantage of a two-antenna swept frequency reception scheme is anangular resolution capability proportional and unambiguous with respectto antenna spacing. This permits the installation of small butwidespaced antennas on aircraft.

in the constant frequency configuration using position-modulatedantennas, unambiguous angular resolution is possible proportional to thefrequency as well as to the position sweep width. Frequently it isnecessary and desirable to utilize aircraft for mapping and in groundsearch operations. Such applications are generally limited by groundclutter. In order to take advantage of resonant features for theenhancement of desired targets one can be forced to use low frequencyradio signals which in turn require large antennas for sufficientangular resolution. Such antennas are impractical if not impossible tobe utilized in currently known aircraft.

Further, this system can find application as a navigational aid foraircraft, functioning in the very same way as it is assumed from bats.An array of two frequency sweep or position sweep systems yieldscomplete and simultaneous angular information about multiple targets inspace. lf, for example, a series of transmitters in a passive groundapproach system were located as markers at spaced intervals along arunway, the nagivation system is capable of telling a pilot by means ofa two-dimensional perspective display his orientation with respect tothe runway, similar to the human eye.

Additionally, an active system utilizing the same concept is possiblewhere signals are transmitted from the aircraft and highly reflectivemembers are located along the runway.

SUMMARY OF THE INVENTION A two-antenna reception system with fixedantenna positions to receive linearly frequency-modulated signals orwith linearly positioned-modulated antennas to receive fixed frequencysignals can be used to determine the angle of arrival of the signals ina plane containing the two antennas and the signal source. This angularinformation is unambiguous within a range of independent of the antennaseparation or relative antenna motion in terms of the wavelengthsemployed The angular resolution of such systems is proportional to thefrequency sweep width and the antenna separation in one case andproportional to the relative antenna motion and the frequency in theother case.

Two orthogonally oriented systems will provide elevation and azimuthinformation.

In general, complete angular information in space is obtained from anytwo systems whose axes are not oriented in parallel. The angularposition of the signal source with respect to the system baselineaffects the time delay or the Doppler shift of the antenna signals inthe fixed position or the linear position-modulation configuration,respectively.

The signal source represents an active transmitter as well as a passivesource (reflector). Since the resolution capabilities of two-antennasystems depend on the antenna spacing or relative antenna motion widthrather than on antenna size, they offer a solution when large-sizeantennas cannot be tolerated in particular applications. If simultaneousprocessing of signals arriving from different directions is provided bymeans of processing a spectrum of discrete difference frequencies, theseschemes are capable of simultaneous acquisition of perspectiveinformation from spatially distributed signal sources. This is incontrast to any kind of electrical or mechanical beam-scanningprocedure.

It is therefore an object of this invention to provide a new andimproved means for finding angular information.

It is another object of the invention to provide a new and improvedsystem to derive unambiguous directional information from a two-antennasystem independent of antenna spacing or variation of this antennaspacing with time.

lt is a further object of the invention to provide a new and improvedsystem which achieves resolution through two wide-spaced or variablyspaced antennas rather than large size antennas.

It is still another object of the invention to provide perspectiveinformation similar to the human eye, using an array of the two-antennasystems.

It is still a further object of this invention to provide means ofsimultaneous acquisition of angular information from multiple targets asopposed to sequential acquisition by mechanical or electrical beamswinging.

It is another object of the invention to provide a multiple antennareceiving system which is economical to produce and which utilizesconventional currently available components that lend themselves tostandard mass production manufacturing techniques.

These and other advantages, features and objects of the invention willbecome more apparent from the following description taken in connectionwith the illustrative embodiments in the accompanying drawings, wherein:

DESCRIPTION OF THE DRAWINGS FIG. 1 is a transmitter-receiver geometryfor the frequency-modulation system;

FIG. 2 is a transmitter-receiver geometry for the position-modulationsystem;

FIG. 3 is a block diagram of the two-antenna signal processing system ofthe invention; and

FIG. 4 is a block diagram of the double two-antenna signal processingsystem of the invention reduced to a three-antenna configuration.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to F lGS. l and2, two antennas 110 and 12 receive a linearly frequency-modulated signal(df/dt constant) from a source 14. By mixing the two antenna signals, adifference frequency Af can be generated which is a function of theangle of arrival a, Af= (s-cos a/c)-(df/dt), where c is the velocity oflight. For a constant frequency signal f transmitted from source and alinear motion of antenna 18 as in FIG. 2 (an tenna l6 fixed) thedifference frequency Af is Af (f-cos a/c)'(ds/dt). Here ds/dt constantis the velocity of antenna 18. When antenna 16 simultaneously moves withthe same velocity in the opposite direction, Af doubles. A frequencysweep width df leads to a system beam width proportional to (A/s)-(f/df)and a relative antenna motion ds to a beam width proportional to Al ds.Here A is equal to c/f.

In FIG. 3 either of the two-antenna systems of FIGS. 1 and 2 arerepresented by antennas 22 and 24 which feed the received signals intothe mixer 26. A difference frequency signal Af as a function of theangle of arrival at is generated in mixer 26 by heterodyning the twoantenna signals according to the equations for Af given above. Amultitude of difference frequencies Af are generated from acorresponding multitude of target signals, if each target signal arrivesat a different angle a. The difference frequencies Af are then detectedin the frequency analyzer 28. This frequency analyzer may be of asimultaneous or sequential processing type, that is, a multitude offrequency selective filters covering the expected range of differencefrequencies or a frequency-tunable single filter covering the same rangemay be employed to detect the difference frequencies. Detected signalsare routed through the frequency/direction converter 30 which convertsthe frequencies into DC voltages proportional to the angle of arrival.The conversion process thus eliminates the non-linear relationshipbetween a and Af. Having converted the angles of arrival intoproportional DC signals, these are displayed in the one-dimensionaldisplay unit 32. The term one-dimensional refers to the fact that asignal arriving from space is characterized only by its angle of arrivalat in a single plane established by the target position and antennas 22and 24. Unit 32 can be a CRT whose horizontal axis is scanned insynchronism with the tuning position at the frequency variable filter orwhile the filter bank is sequentially interrogated. If a signal appearsat a particular frequency, its angular direction is displayed on thevertical axis by a corresponding height of the vertical deflection. Thevertical axis can be linearly calibrated in degrees or radians.

Similarly, FIG. 4 depicts the block diagram of a set of two such systemsexplained in FIG. 3 which provide complete information on the spatialangular position of a target while measuring angles of arrival a, and ain two different but intersecting planes. instead of establishing theseintersecting planes by the target position, common to both planes, andtwo appropriate pairs of antenna positions 22 and 24 of FIG. 3, oneantenna 36 can be used jointly as indicated in FIG. 4. The twointersecting planes in which the angles of arrival are measured are nowdefined by the target position and the position of antennas 34 and 36respectively by the target position and the position of antennas 36 and38.

The signal processing in mixers 40 and 42 is analogous to that in mixer26, likewise frequency analyzers 44 and 46 correspond to frequencyanalyzer 28. DC voltages proportional to the angles of arrival in thespecified planes are derived in the frequency/direction converters 48and 50 as outlined for unit 30. DC voltages corresponding to a, and 11are then displayed in a two-dimensional display unit 52 which in itssimplest form consists of two units 32, but may be truly twodimensionalin that a pair a and a derived from an individual target represents thehorizontal and vertical coordinates of a point on the CRT. The termtwodimensional refers to the fact that a signal arriving from space ischaracterized by two angles measured in nonparallel planes whichuniquely define the spatial direction of the target.

Obviously, many modifications and variations of the present inventionare possible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

I claim:

l. A multiple antenna direction finding system comprising: a first andsecond antenna moving with a known velocity relative to each other forreceiving fixed frequency signals from a target; a signal mixing meansconnected to each antenna for obtaining a difference frequency signalfrom the output of the first and second antenna; frequency analyzingmeans for identifying a particular frequency from a spectrum of possiblefrequencies at the mixer output; means to convert the identifiedfrequency information into angular information; and a calibrated meansfor displaying onedimensional directional information operativelyconnected to thfi converter means.

1. A multiple antenna direction finding system comprising: a first andsecond antenna moving with a known velocity relative to each other forreceiving fixed frequency signals from a target; a signal mixing meansconnected to each antenna for obtaining a difference frequency signalfrom the output of the first and second antenna; frequency analyzingmeans for identifying a particular frequency from a spectrum of possiblefrequencies at the mixer output; means to convert the identifiedfrequency information into angular information; and a calibrated meansfor displaying one-dimensional directional information operativelyconnected to the converter means.